of Sirius, and,
in fact, indicated an exact place where the companion ought to be. The
obscure companion of Sirius became now a recognized celestial object,
although it had never been seen, and it was held to revolve round Sirius
in fifty years, and to be about half as big.

In 1862, the firm of Alvan Clark and Sons, of New York, were completing
a magnificent 18-inch refractor, and the younger Clark was trying it on
Sirius, when he said: "Why, father, the star has a companion!" The elder
Clark also looked, and sure enough there was a faint companion due east
of the bright star, and in just the position required by theory. Not
that the Clarks knew anything about the theory. They were keen-sighted
and most skilful instrument-makers, and they made the discovery by
accident. After it had once been seen, it was found that several of the
large telescopes of the world were able to show it. It is half as big,
but it only gives 1/10000th part of the light that Sirius gives. No
doubt it shines partly with a borrowed light and partly with a dull heat
of its own. It is a real planet, but as yet too hot to live on. It will
cool down in time, as our earth has cooled and as Jupiter is cooling,
and no doubt become habitable enough. It does revolve round Sirius in a
period of 49.4 years--almost exactly what Bessel assigned to it.

But Bessel also assigned a dark companion to Procyon. It and its
luminous neighbour are considered to revolve round each other in a
period of forty years, and astronomers feel perfectly assured of its
existence, though at present it has not been seen by man.

LECTURE XV

THE DISCOVERY OF NEPTUNE

We approach to-night perhaps the greatest, certainly the most
conspicuous, triumphs of the theory of gravitation. The explanation by
Newton of the observed facts of the motion of the moon, the way he
accounted for precession and nutation and for the tides, the way in
which Laplace explained every detail of the planetary motions--these
achievements may seem to the professional astronomer equally, if not
more, striking and wonderful; but of the facts to be explained in these
cases the general public are necessarily more or less ignorant, and so
no beauty or thoroughness of treatment appeals to them, nor can excite
their imaginations. But to predict in the solitude of the study, with no
weapons other than pen, ink, and paper, an unknown and enormously
distant world, to calculate its orbit when as yet it had